1. Field of the Invention
The present invention relates to a sheet processing apparatus that corrects a curl of a sheet and to an image forming system equipped with the sheet processing apparatus.
2. Description of Related Art
Image forming by an electrophotographic process is one for forming an image through a process wherein a toner image is formed by charging, light exposure and developing, then, the toner image thus formed is transferred onto a sheet, and the toner image thus transferred onto the sheet is fixed, as is known widely.
In a fixing step in this process, toner is fused by heat and pressure so that an image is fixed on a sheet, and in this occasion, there is presented a phenomenon that moisture is vaporized by heat from the sheet.
There is further presented a phenomenon that a sheet absorbs moisture from the open air when the sheet is exposed to the open air.
Since a rate of occurrence of these evaporation, absorption and radiation of moisture on a surface of the sheet is different from that on the other side of the sheet, there is a problem that waviness and curls are caused on the sheet.
Sheets having waviness and curl cause troubles for conveyance, processing and stacking in a sheet processing apparatus that is connected to an image forming apparatus, and they become obstacles for processing and storage for sheets such as an increase in a volume of sheets bundled by binding processing, thus, a technology to correct sheet curls has been developed.
Namely, Unexamined Japanese Patent Application Publication No. 4-338060 and Unexamined Japanese Patent Application Publication No. 5-309971 disclose a technology to correct curls by applying mechanical bending force for a sheet on a sheet conveyance path, and Patent Document 2 discloses a technology to select an air volume of a fan in accordance with a sheet type and image density and to radiate moisture of a sheet.
Though an extent of curl correction can be selected in accordance with a sheet type in the structure described in Unexamined Japanese Patent Application Publication No. 4-338060, it has been found that curls cannot be corrected sometimes by adjustment of mechanical bending force alone even if bending force is adjusted, because sheets are of various types.
For example, it has been found that curls can be corrected by applying strong force in the case of a thick sheet, but, in the case of a thin sheet, it is difficult to correct curls by mechanical bending force alone, and it is difficult to correct curls by bending force alone in the case of fine-quality sheets for printing.
It has been further found that curls cannot be corrected by radiating moisture of a sheet alone, in a similar manner, in the structure described in Unexamined Japanese Patent Application Publication No. 5-309971.